Apparatus for railway train communication systems



Aug. 10, 1937. P. N, BossART 2,089,824

A APPARATUS FOR RAILWAY TRAIN COMMUNICATION SYSTEMS Filed April- 7, 1936v 'NQ/R@ HIS ATTORNEY Patented Aug. 10, 1937 UNITED STATES APPARATUS FORRAILWAY TRAIN COM- MUNICATION SYSTEMS Paul N. Bossart, Cheswick, Pa.,assignor to The Union Switch & Signal Company, Swissvale, Pa., acorporation of Pennsylvania,

Application April 7, 1936, Serial No. 73,135

12 Claims.

My invention relates to apparatus for railway train communicationsystems, and particularly to telephone systems for railway trains.

I will describe one form of apparatus embody- 5 ing my invention, andwill then point out the novel features thereof in claims.

A feature of my invention is the provision of novel and improvedapparatus for systems of the type here under consideration wherewith anoperator at the station receiving a telephone message may break in uponthe conversation and signal to the operator talking. Another feature ofthe invention is the provision of apparatus of the Ytype here involvedwherewith receipt of a call- 15 ing current for a very brief intervalduring a conversation automatically takes over control of the apparatus,so that as long as the other station continues to send a calling currentit is received at the station being broken in upon.

Other features and advantages of my invention will appear as thespecification progresses.

In railway train communication systems of the .type here contemplated, acarrier current modulated with voice frequencies is used for telephone25 communication, and a current equal to such carrier modulated by asingle predetermined voice frequency is used for calling. Atrain-carried station in sending out current supplies the current to acommunication channel which Vpreferably includes the track rails, thecurrent being supplied to the channel through the medium of a sendingcircuit effectively coupled with the track rails. Current is received ata train-carried station through the medium of a receiving circuit feffectively coupled with the track rails. The receiving apparatus ateach station is normally active and in circuit ready to receive amessage from the remote station at any time. The transmitting apparatusat each station is normally inactive and is energized and conditionedfor supplying current only during periods a current is to be sent outfrom that station. Switching apparatus is provided for selectivelyrendering either the transmitting apparatus or the receiving apparatusoperative and effectively coupled to the track rails. In systemsheretofore proposed, the operator actuates such switching apparatusmanually. As will appear hereinafter, I provide a voice operated deviceor relay which cooperates with the manually operated mechanism inswitching the connections between the transmitting and receivingapparatuses. Such voice operated relay automatically switches thereceiving apparatus into operation during each pause in a conversation,and cuts out the receiving apparatus on the rst syllable of a group ofwords. Receipt of an incoming calling current during such pause of theoutgoing conversation eectively influences the receiving apparatus andresults in a visible or audible signal being displayed or sounded to 5indicate to the operator talking that the operator at the remote stationdesires to break in upon his conversation. The receiving apparatus thusinfluenced also renders the microphone ineffective and the operator atthat station can no longer transmit his conversation. In other words,apparatus .embodying my invention provides the equivalent of a duplextelephone system for railway trains.

The accompanying drawing is a diagrammatic view of a preferredembodiment of my invention.

The receiving apparatus will preferably include an electron tubeamplier-demodulator, a loudspeaker or telephone receiver, and a callresponsive circuit effective when energized to operate 20 a signalingdevice. The transmitting apparatus will preferably include a source ofcarrier current such as an electron tube oscillator, a modulator-ampliercapable of suppressing the carrier and selecting a desired side band, apower amplifier such as a pair of power electron tubes connected in theusual push-pull arraement, and calling means capable of supplying afrequency equivalent to the carrier modulated at a single predeterminedvoice frequency. The electron tube oscillator, amplifiers, modulator,and demodulator will be described in the present application only in sofar as is deemed necessary for full understanding of my invention, sincethe specific structure of such devices forms no part of my invention. Tolx the ideas, I shall assume that a band of voice frequencies extendingfrom 500 to 2500 cycles per second for telephoning is desired, thecalling voice frequency is 800 cycles per second, a carrier of '7000cycles per second is employed, and single side band transmission iscontemplated, the upper side band being utilized. It follows that duringtelephone communication a band of frequencies extending from 7500 to9500 cycles is transmitted, and for calling the frequency is '7800cycles. It will be understood, of course, that my invention is notlimited to these specific frequencies and other frequencies may be usedif found desirable.

A power unit for supplying the proper plate voltages to the variouselectron tubes is provided. As here shown, the arrangement is such thatthe power unit is rendered active as needed and the generated voltage isswitched to the plate circuits of the transmitting and receiving tubesaccording to Whether communication current is to be transmitted or is tobe received. Consequently, the transmitting apparatus and the receivingapparatus. of a location are selectively rendered active and effectivelycoupled to the track rails by switching plate voltage to the electrontubes of the respective apparatuses.

Referring to the drawing, the refe-rence character V designates avehicle of `a-railway train,

such as, for example the caboose of a freight train and on which vehiclecommunication equipment is located. The transmitting apparatus isindicated as a whole by the reference character TA, the receivingapparatus by the reference character RA, and the power unit at PU. Theoscillator, modulator and single side band amplifier of the transmittingapparatus is indicated at OMA, and the power amplifier at PA, thesedevices being shown conventionally for the sake of simplicity since, asstated hereinbefore, their specific structure forms no part of myinvention. Device OMA includes anelectron tube oscillator capable ofsupplying a carrier current ot 7000 cycles per second, means to modulatesuch carrier current at voice frequencies and to suppress the carrierand one side band,l the remaining side band being supplied tothe inputot the power amplier PA where it is amplified to a relatively highenergy level.v As here shown, the carrier current is modulated by voicefrequencies produced in a microphone M connected with the input of thedevice OMA over wires 2! and 22, a front contact 23 of a relay R2 to bereferred to later being interposed in the wire k22. The output of thedevice OMA isvsupplied to the inputrof the power amplifier PA over wireslll and H, and the output of the power amplier PA is, in turn,l suppliedover Wires l2 and i 3 to the primary winding lli of a transformer Ti.The secondary winding l5 of transformer Tl is interposed in a sendingcircuit for the vehicle 'V and which circuit extends from the topterminal of winding l5 over wire I6 to a truck wheel l1 at theright-hand end of the vehicle V, track rails I8, truck wheel I9 at theleft-hand end of vehicle V, and wire 20 to the lower terminal of thesecondary winding l5, connection with the wheels il and I9 beingpreferably made through their journal bearings. It follows thatVoiceffrequencies produced by speaking into the microphone M aretranslated to the upper side band of the carrier at the device OMA,further amplified at the amplifier PA, and then supplied to thek sendingcircuit for the vehicle Vr with the resultlthat such communicationcurrent is transmitted along the track rails in both directions fro-mthe vehicle in the well known manner.

As here shown, the microphone M is mounted on a handle 24 to facilitateits being brought close to the person speaking. Two push buttons 25 and26 of the common spring return type are mounted on the handle 24 asindicated by dotted lines. Depressing' the push-button l25 to close itscontact 2 completes a connection between wires 28 and 29 leading to thedevice OMA, and causes that device to producea calling current rwhich inthis instance is `of '7,800 cycles per second. YSuch `calling frequencymay be effected by changing the capacitance of the-associated electrontube oscillator. This calling-frequency current when thus producedispassedv over wires ii) and li to the power amplifier PAsand issupplied to the track rails through the sending circuit in the samemanner as described for the telephonecurrent. Depressing the push button26 to close its contact 30 completes a simple circuit for energizing arelay DR as will be readily understood by an inspection of the drawing.The function of relay DR will be referred to later in the description.

An inductor coil CI is mounted on the vehicle V in inductive relationwith the track rails and is connected over wires 3l and 32 with theinput of the amplifier-demodulator AD of the receiving apparatus RA.This amplifier-demodulator AD is shown conventionally for the sake ofsimplicity since, as stated above, its specific structure Vforms no partof my invention. It follows that communication current flowing in thetrack rails will induce electromotive fonce in the inductor Cl whichelectromotive force will be impressed upon the input of theamplifier-demodulator AD, and the corresponding `audio frequencies willappear at the loutput terminals 33 and 34 of the device AD. These outputterminals 33 and 34 are connected with a loud-speaker circuit or with acall responsive circuit. The loudspeaker circuit may be traced from theterminal 33 over wire 35, operating winding of a loudspeaker LS, wire33, back contact 37 of relay DR, and wire 38 to terminal 34. The callresponsive circuit extends from terminal 33 through condenser 39,reactor 0, wire lli, front contact i2 of relay DR, and wire 38 toterminal 313. A portion of reactor 4Q is connected across the inputterminals of a full wave rectifier 53, the output terminals of which areconnected with the winding of a relay RI. This call responsive circuitis so proportioned and adjusted that it is tuned to resonance at thevoice frequency of 800 cycles. Thus with relay DR picked up and itsfront contact 42 closed to complete-the call responsive circuit and acalling current is received by the inductor Ci, the relay Rl isenergized. Under normal conditions, that is, with the apparatus in theposition illustrated, the

loud-speaker circuit is completed at the back contact 3l of relay DR andthe voice frequencies of a telephone current received by the inductor CIare reproduced rat the loud-speaker LS, and a calling current receivedby the nductorr Ci is re- TheU motor generator DM! is constantly activeby virtue of its motor 44 being continuously supplied with current fromany convenient source of current such as a battery not shown, the twoterminals of such source being indicated .at B32 and N32.l AThe motorgenerator DM2 is normally inactive and is set into operation by currentsupplied to the motor 45 when relay DR is picked up land its frontcontact 43 is closed. The generator i? of y motor generator DM! isconnected with the plate circuits for the electron tubes of theamplier-demodulator AD, ythe circuit extending from the positiveterminal of generator 47 over wire 8, back contact le of a relay VOR tobe referred to later, wire 53, plate circuits for the device AD, andwire 5l to the negative terminal of generator 4l. The generator 47 isconnected with the plate circuits for the tubes of theoscillator-modulator-amplifier OMA by a circuitextending from itspositive terminal over wires 48 yand 52, front contact 53 of relay DR,wire 5t, plate circuits of device- OMA, and wire to the negativeterminal of generator 4l. Energization of relay VOR in a manner toshortly appear to close its front contact 5t connects the two generators41 and 51 in series with the plate circuits of the tubes of poweramplier PA, the circuit being traced from the positive terminal ofgenerator 41 over wire 48, front contact 56 of relay VOR, generator 51,wire 58, plate circuits for the tubes of the power amplier PA, and wires59 and 55 to the negative terminal of generator 41. It is clear,therefore, that with relays DR and VOR both deenergized, the motorgenerator l DMI only is active and plate voltage is applied to the tubesof the receiving apparatus; with relay DR picked up and relay VORdeenergized, both motor generators are active and plate voltage isapplied to the tubes of the device OMA of the transmitting apparatus andalso to the tubes of the receiving apparatus; and `with both relays DRand VOR picked up, the voltage of the generator 41 is applied to thetubes of the device OMA of the transmitting apparatus and voltage of thetwo generators 41 and 51 in series is applied to tubes of the poweramplier PA.

The relay VOR is a voice operated relay and is controlled by rectifyinga portion of the single side band energy supplied from the device OMA tothe input of the power amplifier PA. 'I'he input terminals of a fullwave rectier 66 are connected across the wires l and l l, and the outputterminals of this reetier are connected with the winding of relay VOR, aresistor 6l being interposed in series with the winding of relay VOR anda condenser 62 being connected in parallel therewith. rIfhe relay VOR isof the usual direct current type proportioned and adjusted forrelatively quick-acting characteristics. 35 The resistor 6l is used forlessening the release period of the relay, and the condenser 62 is usedto increase the release period. By proper choice of condenser 62 andresistor 6I, and by proper proportioning of the parts of relay VOR, the40 speed of operation of relay VOR is readily adjusted as desired. Forexample, a pause of 0.1 second very frequently occurs in a conversation,and relay VOR is adjusted to release at something less than, say, 0.1second. The pick-up 45 time of the relay would be, say, 0.02 second. Itfollows that with the device OlVIA active and voice frequencies producedby speaking into the microphone M, the relay VOR is picked up duringspeaking periods, is released to close its back 50 contacts duringnormal pauses that occur at the end of a phrase in a conversation, andis then again picked up on the rst syllable of the next word group.

In describing the operation of the apparatus I 55 shall assume that theapparatus is in the position illustrated in the drawing and the operatoron the vehicle V desires toI establish telephone communication, and tothat end he first depresses the push button 26 to pick up the relay .60DR to close its front contacts 46 and 53 to thereby start up the secondmotor generator DMZ and to apply plate voltage to the tubes of thedevice OMA. The operator would ordinarily first transmit a callingcurrent by depressing push button25, 65 A portion of the calling currentenergy delivered by the device OMA to the power amplifier is rectifiedand energizes the relay VOR with the result that its front contact 56 isclosed and plate voltage is applied to the power amplier PA and 70 thecalling current is supplied to the track rails. It is to be noted thatwith relay DR picked up and its front contact 42 closed, the callresponsive circuit is completed but the receiving apparatus RA isinoperative due to plate voltage 75 being removed from the tubes of thereceiving apparatus at the back contact 49 of relay VOR. After a shortinterval the operator would discontinue the calling current by releasingpush button 25, and would initiate his conversation by speaking into themicrophone M. A portion of the side band telephone current delivered bythe device OMA to the power ampliiier PA is rectied and used to energizerelay VOR which relay picks up on the first syllable of a spoken wordand applies plate Voltage to the tubes of the amplifiers PA so that theside band telephone current is supplied to the rails. Ordinarily theoperator of vehicle V carries on his conversation with the remoteoperator by depressing push button 26 when he wishes to speak andreleasing the push button when he wishes to listen. The operator byholding the push button 25 depressed may continue his conversation aslong as desired. When a pause occurs at the end or a phrase in theconversation of the operator of vehicle V, however, the relay VOR isreleased and the receiving apparatus is rendered operative and the poweramplifier PA is rendered inoperative. If the operator at the remotestation desires to break in upon the conversation of the operator ofvehicle V, he would close his push buttons 25 and 26 and eiect thesending of a calling current from the remote station. Under suchcircumstances and with relay VOR released at the pause which occurs atthe end of the neXt phrase in the conversation of the operator ofvehicle V,

the incoming calling current eiectively influl ences the receivingapparatus and energizes the relay RI since, as explained above, the callresponsive circuit is now closed at iront contact 42 of relay DR. Withrelay Rl energized, the circuit for relay R2 is opened at back contact63 and relay R2 is released and at its back contact 64 completes acircuit for a signaling device 65 which is here shown as an electricbell. Relay R2, upon releasing, also opens the microphone circuit at itsfront contact 23. Thus, the bell 65 is sounded and further sending oftelephone current from vehicle V is prevented as long as the callingcurrent is received to retain relay Rl energized. The operator of thevehicle V upon receiving such signal will know that the other operatorat the remote station desires to talk to him at once and will set hisequipment in condition to receive a telephone message by releasing thepush button 26.

It is to be noted that relay VOR is energized only during periods wheneither a calling current or a telephone current is being sent fromvehicle V, since the carrier current is suppressed within the device OMAand upper side band energy only passed to amplier PA. Hence, with relayDR picked up and device OMA rendered active, the relay VOR remainsdeenergized until either the push button 25 is depressed to produce acalling current or the operator speaks into the microphone M.

" An alternative method of controlling the receiving apparatus would beto short-ciicuit the wires 3| and 32 at a front contact 65 of the voiceoperated relay VOR. Such short-circuit is shown on the drawing by thedotted lines 61 and 68V which connects the front contact 66 of relay VORacross the wires 3| and 32. It may not be necessary to remove the platevoltage from the tubes of the power amplier PA during pauses in theconversation, but this is done to avoid the danger of feed back causingsustained tones at the loud-speaker or operating the call responsivecircuit.

It will be understood, of course,.that ripple iilters, voltage adjusting.resistors and spark suppression condensers would be provided in theusual rnanner but are not shown on the drawing for the sake ofsimplicity.

Although I have herein shown and described only one form of apparatusembodying my in- Vention, it is understood that various changes andmodifications may be made therein within the scope of the appendedclaims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is: 1 l. In a railwaycommunication system consisting of train-carried transmitting apparatuscapable of supplying a side band carrier telephone current,train-carried receiving apparatus capable of reproducing the audiofrequencies oi such carrier telephone current and a communicationcircuit including the track rails the combination with the aboveinstrumentalities comprising; a relay, means to connect a winding ofsaid relay with the output side of the modulator of the transmittingapparatus toenergize the relay with a portion of the modulated carrierenergy, said relay characterized by being quick acting whereby the relayis released during a pause in a telephone conversation and is picked upon the iirst syllable of a spoken word, means including a front contactof said relay to effectively couple the transmitting apparatus t0` thetrack rails, and means including a back contact of the relay toeffectively couple the receiving apparatus to the track rails.

2. In a railway communication system consisting of train-carriedtransmitting apparatus capable of supplying a side band carriertelephone current, train-carried receiving apparatus capable ofreproducing the audio frequencies of such carrier telephone current anda communication circuit including the track rails the combination withthe above instrumentalities comprising; a relay, means including arectifier to connect a winding of said relay with the output side of themodulator of the transmitting apparatus to energize said relay with aportion of the voice modulated carrier energy, means including aresistor .and a condenser associated with said relay to provide therelay with quick acting characteristics whereby the relay is releasedduring a pause in a telephone conversation and is picked up on the iirstsyllable of a spoken word, means including a front contact of said relayto effectively couple the transmitting apparatus tothe track rails, andmeans including a back contact of said relay to effectively co-uple thereceiving apparatus with the track rails.

3. In a railway communication system consisting of train-carriedtransmitting apparatus including an electron tube modulator-amplier andpower ampliiier for supplying a carrier telephone current, receivingapparatus including an electron tube amplier-demodulator for reproducingthe telep-hone message of such carrier telephone current, a power unitto supply plate voltage to the electron tubes and a lcommunicationcircuit coupled with both the transmitting and receiving apparatus thecombination with the above instrumentalities comprising, a relay, meansto connect a winding of the relay with the modulator-amplifier toenergize the relay with a portion of the voice modulated carrier energy,means to connect the power unit with the power amplifier and including afront contact of said relay, and means to connect the 'power unit withthe amplier-demodulator and including aback c ontact of said relay. A

4. In a railway communication system consisting of train-carriedtransmitting apparatus including an electron-tube modulator-amplier andpower amplifier for supplying a carrier telephone current, receivingapparatus including an electron tube amplier-demodulator for reproducingthe telephone message of such carrier telephone current, a power unit tosupply plate voltage to the electron tubes and a communication circuitcoupled with both the transmitting and receiving apparatus thecombination with the above instrumentalities comprising, a first and asecond relay, means to connect the power unit with themodulator-ampliiier and including a front contact of the first relay,means to connect the power unit with the power ampliiier and `includinga front contact of the second relay, means to connect the power unitwith the ampliiier-demodulator and including a back contact or" thesecond relay, manually controlled circuit Ameans including a currentsource for energizing the iirst relay, and other circuit means toconnect a winding of the second relay with the modulator-amplifier toenergize the relay with a portion ofthe voice modulated carrier energy.

5. In a railway communication system consisting of train-carriedtransmitting apparatus capable kof supplying a carrier telephone currentand a calling current equivalent to the carrier modulated by a singlepredetermined voice irequency, receiving apparatus capable ofdemodulating such telephone and calling currents and a Vcommunicationcircuit including the track rails in combination with the aboveinstrurnent'alities comprising, a relay, means including a rectifier toconnect a winding of theI relay with the transmitting apparatus toenergize the relay with a portion of the modulated carrier energy, means,including a iront contact of the rel-ay to effectively couple thetransmitting apparatus with the .track rails, means including a Vbackcontact of the relay to effectively couple the receiving apparatus withthe track rails, a call responsive circuit connected with the receivingapparatus and selectively responsive to said predetermined voicefrequency, and means controlled by said call responsive circuit forrendering the transmitting apparatus ineiective to supply telephonecurrent. f

6. In a railway communication system consisting of train-carriedtransmitting apparatus capable of supplying a carrier telephone currentand a calling current equivalent to the carrier modulated by a singlepredetermined voice frequency, receiving apparatus capable ofdemoduvlating such telephone and calling currents and Aa communicationcircuit including the track rails in combination with the aboveinstrumentalities comprising, va relay, `means including a rectier to4connect a winding of the relay with the transxmitting apparatus toenergize the relay with a portion kof `the modulated carrier energy,said relay characterized by being quick acting whereby the relay isVreleased during a pause in a telephone conversation and is picked up onthe rst syllable of a spoken word, means includ- .ing a front contact ofthe relay to effectively coulple the transmitting apparatus with thetrack rails, means includinga back contact of the relay to effectivelycouple the receiving apparatus with the track rails, a call responsivecircuit connected with the receiving apparatus and selectivelyresponsive-to said predetermined-voice frequency, a signaling devicecontrolled by said call responsive current, and other means controlledby said Vcall responsive circuit when energized yfor rendering thetransmitting apparatus ineffective' to supply telephone current.

7. A railway communication system comprising, train-carriedtransmittingapparatus including an oscillator-modulator-amplier, acircuit including a microphone connected with theoscillator-modulator-amplifier to produce a carrier telephone current, acircuit connection including a manually operated contact connected withthe oscillator-modulator-amplier and effective when closed to produce acalling current equivalent to the carrier modulated with a singlepredetermined voice frequency, train-carried receiving apparatusincluding an amplier-demodulator capable of demodulating such telephoneand calling current, means to effectively couple both the transmittingapparatus and the receiving apparatus with the track rails fortelephoning communication with a remote station, a call responsivecircuit connected to the ampliiier-demodulator and selectivelyresponsive to Said predetermined voice frequency, a signaling devicecontrolled by said call responsive circuit when energized, and othermeans controlled by said call responsive circuit when energized forrendering the circuit including the microphone ineffective.

8. A railway communication system comprising, train-carriedtransmitti-ng apparatus including an oscillator-modulator-amplier, acircuit including a microphone connected with theoscillator-modulator-amplifier to produce a carrier telephone current, acircuit connection including a manually operated contact connected withthe oscillator-modulator-ampliier .and effective when closed to producea calling current equivalent to the carrier modulated with a singlepredetermined voice frequency, train-carried receiving apparatusincluding an amplier-demodulator capable of demodul-ating such telephoneand calling currents, a relay, means to connect a winding of the relaywith the transmitting apparatus to energize the relay with a portion ofthe modulated carrier energy, means including a front contact of therelay to effectively couple the transmitting apparatus with the trackrails, means including a 'back contact of the relay to effectivelycouple the receiving apparatus with the rails, a call responsive circuitconnected with the receiving apparatus and selectively responsive tosaid predetermined voice frequency, a signaling device controlled bysaid call responsive circuit, and other means controlled by said callresponsive circuit when energized to render the circuit including themicrophone ineffective.

9. A railway communication system comprising transmitting apparatus andreceiving apparatus mounted on a vehicle of a train, said transmittingapparatus including an oscillator-modulator of the balanced type,coupling means effective at times to couple the output side of theoscillator-modulator with the track rails, said receiving apparatusincluding an amplifier-demodulator the input side o-f which iseffectively coupled with the track rails, a microphone connected acrossthe input side of said oscillatormodulator to cause a side band carriertelephone current to be supplied to the rails by speaking into themicrophone, a loud speaker connected across the output side of saidampliiler-demodulator to reproduce the voice frequency of such carriertelephone current owing in the rails, an electrorespcnsive deviceconnected across the output side of said-oscillator-modulator andeffectively energized by a portion of the telephone current, circuitmeans including a manually operated contact to render saidoscillator-modulator active, means including a contact governed by saiddevice and closed only when the device is deenergized to render. saidamplier--demodulator active, and means including a contact governed bysaiddevice and closedfonly when the device is energized to render saidcoupling means effective.

10. A railway communication system comprising transmitting apparatus andreceiving apparatus mounted on a vehicleof a train, said transmittingapparatus including an oscillator-modulator of the balanced type and apower amplifier the output side of which amplifier is coupled with thetrack rails and the input side of which is connected over a pair ofwires with the output of the oscillator-modulator, said receivingapparatus including an amplier-demodulator the input side of which iscoupled with the track rails, a microphone connected across the inputside of the oscillator-modulator to cause an amplified side band carriertelephone current to be supplied to the rails by speaking into themicrophone, a loud speaker connected across the output side of saidamplier-demodulator to reproduce the voice frequencies of such telephonecurrent flowing in the rails, an electroresponsive device connectedacross said pair of wires and effectively energized when voice frequencyenergy is passed to the power amplifier, a first circuit means includinga manually operated contact to render the oscillator-modulator active, asecond circuit means including a contact governed by said device andclosed only when the device is deenergized to render theamplier-demodulator active, and a third circuit means including acontact governed by said device and closed only when the device isenergized to render said power amplifier active.

1l. A railway communication system comprising transmitting apparatus andreceiving apparatus mounted on a vehicle of a train, said transmittingapparatus including an oscillatormodulator of the balanced type and apower amplifier as well as a connection between the output terminals ofthe modulator and the input terminals of the power amplifier, saidreceiving apparatus including an amplier-demodulator, coupling means foreffectively coupling the output side of said power amplifier and theinput side of said amplier-demodulator with the track rails, amicrophone connected with the input side of said oscillator-modulator tosupply to the rails a side band carrier telephone current, a loudspeaker connected across the output side of said amplier-demodulator toreproduce the voice frequencies of such telephone current flowing in therails, a quick acting relay, means to effectively connect a winding ofsaid relay across the connection between the oscillator-modulator andthe power amplier to pick up the relay in response to the rst syllableof a word spoken into the microphone and to permit the release of therelay during a pause which occurs at the end of a phrase in theconversation, a power unit, means to connect the power unit with thepower amplier and including a front contact of said relay, and means toconnect the power unit with said amplier-demodulator and including aback contact of said relay.

- pabl'e of demodul'ating such telephone and calling currents, anelectrore'sponsive device, means to connect said device with the outputside of said' oscillator-modulator to energize the device With` aportion of the telephone or calling current, means controlled by saiddevice when energized to eiectively couple the transmitting apparatuswith the track rails, means controlled by said device when deenergizedto effectively couple the receiving apparatus with track rails, a loudYspeaker and a call responsive circuit connected With the output side ofsaid amplier-demodu lator, said call responsive circuit selectivelyresponsive to said predetermined voice frequency, and a signalcontrolled by said call responsive circuit.

PAUL N. BOSSART.

